The present invention relates generally to stepping motor control circuits and more particularly to circuits for controlling the energization of stepping motors in less than full steps.
In stepping motors it is common to energize the field windings of a motor in a sequential step-by-step manner which will cause the armature of the motor to rotate in a correspondingly step-by-step manner. Additionally, a number of circuits have been designed to drive a stepping motor in half steps. To improve stepping resolution, U.S. Pat. No. 3,445,741 to Gerber discloses a technique for producing fractional steps to control a stepping motor. This system requires that for producing N sub-steps in a four phase stepping motor, 2N output transistors and associated logic circuitry must be utilized. Thus, in order to produce very fine steps, this circuit becomes quite complex. The control logic forces the formation of a trapezoidal waveform, composed of steps of equal amplitude and equal duration.
U.S. Pat. No. 3,885,210 to Burnett and U.S. Pat. No. 3,909,693 to Yoshitake et. al., disclose stepping motor drive circuits which form a trapezoidal waveform wherein the steps are of unequal amplitude but of equal duration. Both the Gerber, the Burnett, and the Yoshitake et. al., stepping motor control circuits drive the motor with trapezoidal current waveforms which produce varying holding torques over the mini-steps within a step. A mini-stepping motor control drive circuit should produce motor motion in steps of equal amplitude with the same holding torque at each step for greater torque capability and rotation smoothness. Since the motor operates in sinusoidal relationships, the current waveforms should preferably be sinusoidal, and not trapezoidal. Since the motor also has non-linearities, the sinusoid must be altered to compensate for these non-linearities. For a further discussion of this problem see Applicant's U.S. Pat. Application Ser. No. 501,891, filed Aug. 30, 1974.